Switchable package capacitor for charge conservation and series resistance

ABSTRACT

In one embodiment, an apparatus comprises a capacitor and a die. The die comprises a resistor switch coupled between a power line and the capacitor, wherein the resistor switch has an adjustable resistance, and the power line and the capacitor are both external to the die. The die also comprises a circuit configured to receive power from the power line, and a controller configured to open the resistor switch if the power line is powered down.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.14/250,150, filed on Apr. 10, 2014, which is incorporated herein byreference in its entirety.

BACKGROUND

1. Field

Aspects of the present disclosure relate generally to packagecapacitors, and more particularly, to switchable package capacitors.

2. Background

A power distribution network (PDN) may be used to distribute power froma power supply (e.g., a battery) to various circuits on a die.Inductance and capacitance in the PDN may cause the impedance of thePDN, as seen by a circuit on the die, to peak at a resonant frequency ofthe PDN. The peak impedance may cause a large voltage ripple to appearon a power rail of the die when the circuit excites the resonantfrequency of the PDN. The ripple may cause circuit elements on the dieto malfunction.

SUMMARY

The following presents a simplified summary of one or more embodimentsin order to provide a basic understanding of such embodiments. Thissummary is not an extensive overview of all contemplated embodiments,and is intended to neither identify key or critical elements of allembodiments nor delineate the scope of any or all embodiments. Its solepurpose is to present some concepts of one or more embodiments in asimplified form as a prelude to the more detailed description that ispresented later.

According to a first aspect, an apparatus is described herein. Theapparatus comprises a capacitor and a die. The die comprises a resistorswitch coupled between a power line and the capacitor, wherein theresistor switch has an adjustable resistance, and the power line and thecapacitor are both external to the die. The die also comprises a circuitconfigured to receive power from the power line, and a controllerconfigured to open the resistor switch if the power line is powereddown.

A second aspect relates to an apparatus. The apparatus comprises acapacitor and a die. The die comprises a resistor switch coupled betweena power line and the capacitor, wherein the resistor switch has anadjustable resistance, and the power line and the capacitor are bothexternal to the die. The die also comprises a circuit configured toreceive power from the power line, and a resistor controller configuredto dynamically adjust the resistance of the resistor switch during powerup of the circuit from an inactive state to an active state.

A third aspect relates to a method for tuning a resistance of a resistorswitch on a die, wherein the resistor switch is coupled between anexternal power line and an external capacitor. The method comprisessequentially setting the resistance of the resistor switch to each oneof a plurality of resistance values, and, for each resistance value,determining a peak impedance for the resistance value. The method alsocomprises selecting one of the resistance values based on the determinedpeak impedances.

To the accomplishment of the foregoing and related ends, the one or moreembodiments comprise the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative aspects ofthe one or more embodiments. These aspects are indicative, however, ofbut a few of the various ways in which the principles of variousembodiments may be employed and the described embodiments are intendedto include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a power distribution network (PDN) fordistributing power to one or more circuits on a die according to anembodiment of the present disclosure.

FIG. 2 shows an example of a die and a package according to anembodiment of the present disclosure.

FIG. 3 shows an example of a multilayer package capacitor according toan embodiment of the present disclosure.

FIG. 4 is a plot showing impedance as a function of frequency accordingto an embodiment of the present disclosure.

FIG. 5 is a plot showing impedance as a function of frequency for twodifferent resistance values according to an embodiment of the presentdisclosure.

FIG. 6 shows an example of a PDN comprising a switchable packagecapacitor according to an embodiment of the present disclosure.

FIG. 7 shows a die and a package with a switchable package capacitoraccording to an embodiment of the present disclosure.

FIG. 8 shows an exemplary implementation of a resistor switch accordingto an embodiment of the present disclosure.

FIG. 9 is a plot showing impedance as a function of frequency for threedifferent switch resistances according to an embodiment of the presentdisclosure.

FIG. 10 shows circuitry for tuning the resistance of a resistor switchaccording to an embodiment of the present disclosure.

FIG. 11 is a plot showing minimum measured voltage as a function ofswitch resistance according to an embodiment of the present disclosure.

FIG. 12 is a flow diagram illustrating a method for tuning theresistance of a resistor switch according to an embodiment of thepresent disclosure.

FIG. 13 shows a die comprising two circuits and a package comprising aswitchable package capacitor according to an embodiment of the presentdisclosure.

FIG. 14 is a plot showing minimum measured voltage as a function ofswitch resistance for different use cases according to an embodiment ofthe present disclosure.

FIG. 15 is a flow diagram illustrating a method for adjusting aresistance on a die according to an embodiment of the presentdisclosure.

FIG. 16 shows voltage droop for two different switch resistancesaccording to an embodiment of the present disclosure.

FIG. 17 shows circuitry for dynamically adjusting the resistance of aresistor switch during power up of a circuit according to an embodimentof the present disclosure.

FIG. 18 shows an exemplary implementation of a voltage detectoraccording to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

A power distribution network (PDN) is used to distribute power from apower supply (e.g., a battery) to various circuits on a die. FIG. 1shows an example of a PDN for supplying power from a power supply 110 toa circuit 145 on a die 120. The PDN supplies power to the die 120 via apower supply line 112 that traverses a printed circuit board (PCB) 114and a package 117. For ease of illustration, a ground line is not shownin FIG. 1. The portion of the PDN corresponding to the PCB 114 mayinclude board inductance modeled as board inductor L_(PCB) and boardresistance modeled as board resistor R_(PCB). The PDN may also include aboard capacitor C_(PCB), which may have a small amount of inductance andresistance modeled as inductor 126 and resistor 124, respectively.

The portion of the PDN corresponding to the package 117 may includepackage inductance and package resistance. The PDN may also include apackage capacitor C_(pkg), which may have a small amount of inductanceand resistance modeled as inductor 134 and resistor 132, respectively.In one embodiment, the package capacitor C_(Pkg) is located within thepackage 117. As a result, a portion of the package inductance modeled asinductor 128 is located between the power supply 110 and the packagecapacitor C_(pkg), and another portion of the package inductance modeledas inductor 138 is located between the package capacitor C_(pkg) and thedie 120. Similarly, a portion of the package resistance modeled asresistor 130 is located between the power supply 110 and the packagecapacitor C_(pkg), and another portion of the package resistance modeledas resistor 136 is located between the package capacitor C_(pkg) and thedie 120.

The die 120 includes a power grid for supplying power to the circuit 145from the power supply 110. The power grid includes resistance modeled asgrid resistor R_(grid) in FIG. 1. The die 120 also includes apower-gating switch 140 for power gating the circuit 145. For example,the power-gating switch 140 may be configured to connect the circuit 145to the power grid when the circuit 145 is active, and disconnect thecircuit 145 from the power grid when the circuit 145 is inactive (e.g.,idle) to conserve power. The circuit 145 has capacitance modeled as diecapacitor C_(die) and resistance modeled as die resistor R_(die), asshown in FIG. 1. The capacitance of the circuit 145 may includecapacitances of transistors (e.g., metal-oxide-semiconductorfield-effect transistors (MOSFETs)) in the circuit 145 andparasitic-wire capacitances in the circuit 145. The resistance of thecircuit 145 may include parasitic-wire resistances in the circuit 145.

It is to be appreciated that FIG. 1 is not intended to show all of thecapacitances, inductances and/or resistances that may be present in thePDN. Rather, FIG. 1 is intended to show capacitances, inductances andresistances in the PDN that facilitate an understanding of embodimentsof the present disclosure discussed below.

FIG. 2 shows a simplified cross-sectional diagram of the die 120, thepackage 117, and the PCB 114 according to an embodiment of the presentdisclosure. The package 117 is mounted on the PCB 114, and the die 120is mounted on the package 117. Electrical connections between the PCB114 and the package 117 are provided by solder balls 220, and electricalconnections between the package 117 and the die 120 are provided bybumps 215.

In the example shown in FIG. 2, the package 117 comprises a top layer222, a bottom layer 226, and a center layer 224 between the top andbottom layers 222 and 226. The top and bottom layers 222 and 226 mayeach comprise one or more glass-epoxy materials (e.g., FR4), and thecenter layer 224 may comprise one or more ceramic materials. The centerlayer 224 may have a thickness that is greater than the thickness ofeach of the top and bottom layers 222 and 226. For example, the centerlayer 224 may have a thickness (e.g., 60 μm) that is two to four timesgreater than the thickness (e.g., 20 μm) of each of the top and bottomlayers 222 and 226.

As shown in FIG. 2, the package capacitor C_(pkg) may be located withinthe center layer 224 of the package 117. For example, the packagecapacitor C_(pkg) may comprise a multilayer ceramic capacitor embeddedin the center layer 224. In this regard, FIG. 3 shows an example of amultilayer ceramic capacitor 305 that may be used for the packagecapacitor C_(pkg). The capacitor 305 comprises a first plurality ofmetal plates 320-1 to 320-4 coupled to a first metal terminal 310 of thecapacitor 305, and a second plurality of metal plates 322-1 to 322-4coupled to a second metal terminal 315 of the capacitor 305. The metalplates may comprise copper, nickel, silver, tantalum, and/or other typeof metal. As shown in FIG. 3, the first plurality of metal plates 320-1to 320-4 are interlaced with the second plurality of metal plates 322-1to 322-4. Each pair of adjacent metal plates may be separated by aceramic layer (not shown) that acts as a dielectric layer between themetal plates. Each ceramic layer may comprise XSR, X7, Y5V and/or otherceramic material.

Returning to FIG. 2, the package 117 also includes vias 232 to 237 forproviding conduction paths through the package 117. Each of the vias232-237 may be formed by creating a hole in one or more layers of thepackage 117, and filling the hole with metal. The vias 232 to 237 haveinductances and resistances contributing to the package inductance andpackage resistance discussed above.

In the example shown in FIG. 2, vias 232, and 234 form a firstconduction path through the bottom, center and top layers 226, 224 and222, respectively, of the package 117. The first conduction path couplesthe power supply line 112 (e.g., metal trace) on the PCB 114 to the die120. Via 232 of the first conduction path may be coupled to the powersupply line 112 by one of the solder balls 220. It is to be appreciatedthat via 232 does not need to be aligned with the respective solderball. For example, if via 232 is not aligned with the respective solderball, then via 232 may be coupled to the respective solder ball by ametal trace (not shown) on or near the bottom surface of the package117. Via 234 of the first conduction path may be coupled to the die 120by one of the bumps 215. If via 234 is not aligned with the respectivebump, then via 234 may be coupled to the respective bump by a metaltrace (not shown) on or near the top surface of the package 117.

Vias 235, 236 and 237 form a second conduction path through the bottom,center and top layers 226, 224 and 222, respectively, of the package117. The second conduction path couples the ground line 212 (e.g., metaltrace) on the PCB 114 to the die 120. Via 235 of the second conductionpath may be coupled to the ground line by one of the solder balls 220.It is to be appreciated that via 235 does not need to be aligned withthe respective solder ball. For example, if via 235 is not aligned withthe respective solder ball, then via 235 may be coupled to therespective solder ball by a metal trace (not shown) on or near thebottom surface of the package 117. Via 237 of the second conduction pathmay be coupled to the die 120 by one of the bumps 215. If via 237 is notaligned with the respective bump, then via 237 may be coupled to therespective bump by a metal trace (not shown) on or near the top surfaceof the package 117.

The first conductive path may be coupled to one of the terminals of thepackage capacitor C_(pkg), and the second conductive path may be coupledto the other terminal of the package capacitor C_(pkg). For the examplewhere the package capacitor C_(Pkg) is implemented with the multilayerceramic capacitor 305, the first conductive path may be coupled to oneof the first and second metal terminals 310 and 315 of the capacitor305, and the second conductive path may be coupled to the other one ofthe first and second metal terminals 310 and 315 of the capacitor 305.In the example shown in FIG. 2, the package capacitor C_(pkg) is coupledbetween the power supply 110 and the die 120.

Inductance and capacitance in the PDN may cause the impedance of thePDN, as seen by the circuit 145, to peak at a certain frequency. Forexample, as shown in FIG. 1, a resistor-inductor-capacitor (RLC) loop152 (also referred to as an RLC tank) may be formed by the packagecapacitor C_(pkg), package inductor 138, package resistor 136, gridresistor R_(grid), die capacitor C_(die) and die resistor R_(die). InFIG. 1, the RLC loop 152 has two arrows indicating that current may flowin the RLC loop 152 in either direction. The RLC loop 152 causes the PDNto resonate at a resonant frequency given by:

$\begin{matrix}{\omega_{0} = \frac{1}{\sqrt{LC}}} & (1)\end{matrix}$

where ω₀ is the resonant angular frequency, L is the inductance of theRLC loop 152, and C is the capacitance of the RLC loop 152. Theimpedance of the PDN, as seen from the circuit 145, may peak at theresonant frequency of the RLC loop 152, as discussed further below.

FIG. 4 shows the impedance 410 of the PDN, as seen from the circuit 145,as a function of frequency. As shown in FIG. 4, the impedance 410 peaksat the resonant frequency of the RLC loop 152 (denoted F_(res)). Theimpedance causes a voltage ripple on the power grid when the circuit 145draws current from the PDN. The amplitude of the ripple is largest atthe peak impedance 412, which may occur when the circuit 145 draws atime-varying current having a frequency at the resonant frequency of theRLC loop 152. It is desirable to minimize the amplitude of the ripple onthe power grid to ensure that logic connected to the power gridfunctions properly, and therefore to minimize the peak impedance 412.The impedance 410 may also include a small local peak 415 due to an RLCloop associated with the board capacitor C_(PCB).

The peak impedance 412 is controlled by the Q factor of the RLC loop152, which is the ratio of the energy stored by the RLC loop 152 overthe energy dissipated by the RLC loop 152. The lower the Q factor, thelower the peak impedance. The Q factor is inversely proportional to theresistance of the RLC loop 152. Thus, the Q factor (and hence peakimpedance 412) can be reduced by increasing the resistance of the RLCloop 152. One approach to increase the resistance of the RLC loop 152 isto increase the resistance of the grid resistor R_(grid). In thisregard, FIG. 5 shows the impedance 510 of the PDN in which theresistance of the grid resistor R_(grid) has been increased compared tothe impedance 410 in FIG. 4. As shown in FIG. 5, the impedance 510 withthe higher grid resistance has a lower peak 512 at the resonantfrequency. However, increasing the grid resistance has the undesirableeffect of increasing the current-resistor (IR) voltage drop across thegrid resistor R_(grid), which reduces the DC supply voltage at thecircuit 145. As a result, the power supply 110 needs to output a highersupply voltage in order to achieve a given supply voltage at the circuit145.

Embodiments of the present disclosure provide a switchable packagecapacitor that allows the resistance of the RLC loop to be controlled toreduce the peak impedance of the PDN without increasing the IR dropbetween the power supply 110 and the circuit 145, as discussed furtherbelow.

FIG. 6 shows a PDN according to an embodiment of the present disclosure.In this embodiment, the die 620 comprises a resistor switch 615 (e.g.,PMOS transistor switch) between the package capacitor C_(pkg) and thecircuit 145. The resistor switch 615 is included in the RLC loop 652associated with the package capacitor C_(pkg). As a result, theresistance of the resistor switch 615 contributes to the resistance ofthe RLC loop 652, and can therefore be used to control the resistance ofthe RLC loop 652. In this regard, the resistor switch 615 may have aprogrammable resistance and the die 620 may further include a resistancecontroller 625 for controlling the resistance of the resistor switch615, and hence the resistance of the RLC loop 652. For example, theresistance controller 625 may adjust the resistance of the resistorswitch 615 to lower the Q factor of the RLC loop 652, and thereforereduce the peak impedance seen by the circuit 145.

The resistance of the resistor switch 615 may be adjusted to increasethe resistance of the RLC loop 652 to reduce the peak impedance withoutincreasing the IR drop between the power supply 110 and the circuit 145.This is because the resistor switch 615 is located in a path 617 that isseparate from the path between the power supply 110 and the circuit 145.As a result, the resistance of the resistor switch 615 does notcontribute to the resistance between the power supply 110 and thecircuit 145. Thus, the resistor switch 615 allows the resistance of theRLC loop 652 to be adjusted without negatively impacting the IR dropbetween the power supply 110 and the circuit 145, and thus withoutreducing the DC supply voltage at the circuit 145.

In FIG. 6, package inductance between the power supply 110 and the die620 is modeled as package inductor L_(pkg) and package resistancebetween the power supply 110 and the die 620 is modeled as packageresistor R_(pkg). Package inductance and package resistance between thedie 620 and the package capacitor C_(pkg) are not shown in FIG. 6 forease illustration. It is to be appreciated that the path 617 for theresistor switch 615 may be connected to the power grid at differentlocations, and is therefore not limited to the exemplary location shownin FIG. 6.

FIG. 7 shows a simplified cross-sectional diagram of the die 620, thepackage 617, and the PCB 114 according to an embodiment of the presentdisclosure. The package 617 is mounted on the PCB 114, and the die 620is mounted on the package 617. Electrical connections between the PCB114 and the package 617 are provided by solder balls 220, and electricalconnections between the package 617 and the die 620 are provided bybumps 215.

The package 617 in this embodiment differs from the package 117 in FIG.2 in that the package capacitor C_(pkg) is not coupled to the die 620 bythe first and second conduction paths, which couple the power line 112and the ground line 212 to the die 620. Instead, the package capacitorC_(pkg) is coupled to the die 620 by separate conduction paths in thepackage 617.

In this regard, the package may include vias 733 to 736 for coupling thepackage capacitor C_(pkg) to the die 620. Vias 733 and 734 form a thirdconduction path through the center and top layers 224 and 222,respectively, of the package 617. The third conduction path is used tocouple a first terminal of the package capacitor C_(pkg) to the die 620.In this regard, via 733 may be coupled to the first terminal of thepackage capacitor C_(pkg) and via 734 may be coupled to the die 620 byone of the bumps 215. If via 734 is not aligned with the respectivebump, then via 734 may be coupled to the respective bump by a metaltrace (not shown) on or near the top surface of the package 617.

Vias 735 and 736 form a fourth conduction path through the center andtop layers 224 and 222, respectively, of the package 617. The fourthconduction path is used to couple a second terminal of the packagecapacitor C_(pkg) to the die 620. In this regard, via 735 may be coupledto the second terminal of the package capacitor C_(pkg) and via 736 maybe coupled to the die 620 by one of the bumps 215. If via 736 is notaligned with the respective bump, then via 736 may be coupled to therespective bump by a metal trace (not shown) on or near the top surfaceof the package 617.

In this embodiment, the third conduction path in the package 617 may beused to couple the first terminal of the package capacitor C_(pkg) tothe resistor switch 615 on the die 620 and the fourth conduction path inthe package 617 may be used to couple the second terminal of the packagecapacitor C_(pkg) to a ground line on the die 620. Alternately, thethird conduction path may be used to couple the first terminal of thepackage capacitor C_(pkg) to the ground line on the die 620 and thefourth conduction may be used to couple the second terminal of thepackage capacitor C_(pkg) to the resistor switch 615 on the die 620. Inanother example, one of the first and second terminals of the packagecapacitor C_(pkg) may be connected to the ground line 212 of the PCB 114by a via (not shown) through the bottom layer 226 of the package 117 andone of the solder balls 220.

The package capacitor C_(pkg) may comprise a multilayer ceramiccapacitor embedded in the center layer 224 of the package 617, and/orother type of capacitor. For example, the package capacitor C_(pkg) maybe implemented with the multilayer ceramic capacitor 305 shown in FIG.3. It is to be appreciated that the package capacitor C_(pkg) is notlimited to being located within the package 617. For example, thepackage capacitor C_(pkg) may be mounted on top of the package 617 nextto the die 620. In this example, the package capacitor C_(pkg) may becoupled to the die by metal traces on the package 617.

As discussed above, the resistor switch 615 may have a programmableresistance for controlling the resistance of the RLC loop 652. In thisregard, FIG. 8 shows an exemplary implementation of the resistor switch615 according to an embodiment of the present disclosure. In thisembodiment, the resistor switch 615 comprises a plurality of switches815-1 to 815-n coupled in parallel. The resistance controller 625controls the resistance of the resistor switch 615 by controlling thenumber of switches 815-1 to 815-n that are turned on. For example, theresistance controller 625 may reduce the resistance of the resistorswitch 615 by turning on more of the switches 815-1 to 815-n, and mayincrease the resistance of the resistor switch 615 by turning on fewerof the switches 815-1 to 815-n. In FIG. 8, terminal 822 of the switch615 may be coupled to the circuit 145 and terminal 825 of the resistorswitch 615 may be coupled to the package capacitor C_(pkg).

The granularity with which the resistance controller 625 can control theresistance of the resistor switch 615 may depend on the number ofswitches 815-1 to 815-n in the resistor switch 615. For example, thegreater the number of switches 815-1 to 815-n in the resistor switch615, the greater the granularity with which the resistance controller625 can control the resistance of the resistor switch 615.

In one embodiment, each switch 815-1 to 815 n may comprise a PMOStransistor, as shown in FIG. 8. In this embodiment, the resistancecontroller 625 may turn on one of the switches 815-1 to 815-n byinputting a logic zero (low voltage) to the gate of the switch, and mayturn off one of the switches 815-1 to 815-n by inputting a logic one(high voltage) to the gate of the switch.

FIG. 9 shows impedance seen by the circuit 145 as a function offrequency for three different resistance settings of the resistorsswitch 615. In particular, impedance 410 is the impedance seen by thecircuit 145 as a function of frequency when the resistance of theresistor switch 615 is low, and impedance 910 is the impedance seen bythe circuit 145 as a function of frequency when the resistance of theresistor switch 615 is high. Impedance 510 is the impedance seen by thecircuit 145 as a function of frequency when the resistance of theresistor switch 615 is between the low and high resistances.

As shown in FIG. 9, when the resistance of the resistor switch 615 islow, the impedance 410 seen by the circuit 145 has a relatively largepeak 412 at the resonant frequency of RLC loop 652. When the resistanceof the resistor switch 615 is between the low and high resistances, theimpedance 510 seen by the circuit 145 has a smaller peak 512 at theresonant frequency. This is because increasing the resistance of theresistance switch 615 increases the resistance of the RLC loop 652. Theincreased resistance reduces the Q factor of the RLC loop 652, which inturn reduces the peak impedance 512 at the resonant frequency, asdiscussed above.

The impedance at the resonant frequency may be further reduced byincreasing resistance of the resistor switch 615. However, increasingthe resistance of the resistor switch 615 further may cause theimpedance seen by the circuit 145 to increase at lower frequencies. Thisis because increasing the resistance of the resistor switch 615 reducesthe ability of the package capacitor C_(pkg) to supply charge to thepower grid to reduce voltage droops on the power grid. As a result, whenthe resistance of the resistor switch 615 is high, the impedance 910seen by the circuit 145 has a larger peak 912 at lower frequencies,which can lead to higher voltage ripples on the power grid at lowerfrequencies. Accordingly, it may be desirable to tune the resistance ofthe resistor switch 615 to achieve a desired reduction in the peakimpedance.

In this regard, FIG. 10 shows circuitry for tuning the resistance of theresistor switch 615 according an embodiment of the present disclosure.The circuitry includes a voltage detector 1030 and a test circuit 1040.The voltage detector 1030 is configured to measure the supply voltage atthe circuit 145. To do this, the voltage detector 1030 may be coupledbetween the power-gating switch 140 and the circuit 145. Alternatively,the voltage detector 1030 may be coupled between the power-gating switch140 and the power grid.

The test circuit 1040 is configured to perform a procedure for tuningthe resistance of the resistor switch 615. In this regard, the circuit145 may be coupled to the power supply 110 by the power-gating switch140. The test circuit 1040 may then instruct the resistance controller625 to sequentially set the resistance of the resistor switch 615 toeach one of a plurality of different resistance values. For eachresistance value, the test circuit 1040 may determine the peak impedanceacross a frequency range based on voltage measurements from the voltagedetector 1030. The test circuit 1040 may then select the resistancevalue corresponding to the lowest peak impedance. Alternatively, thetest circuit 1040 may select any one of the resistance valuescorresponding to a peak impedance that is equal to or below apeak-impedance threshold. The peak-impedance threshold may be animpedance corresponding to a minimum supply voltage that is needed byone or more circuits on the die to function properly, as discussedfurther below. The test circuit 1040 may then instruct the resistancecontroller 625 to set the resistance of the switch resistor 615 to theselected resistance value.

It is to be appreciated that the peak impedance determined for eachresistance value is not necessarily the absolute peak impedance for theresistance value. For example, the impedance for a particular resistancevalue may be determined for each one of a plurality of differentfrequencies. In this example, the frequency at which the absolute peakimpedance for the resistance value occurs may not exactly match one ofthe plurality of frequencies, in which case the determined peakimpedance approximates the absolute peak impedance.

The test circuit 1040 may determine the peak impedance for a particularresistance value according to the following procedure. In this regard,the circuit 145 may be coupled to the power supply 110 by thepower-gating switch 140 and operated across a frequency range ofinterest. As the circuit 145 is operated across the frequency range, thevoltage detector 1030 may measure the supply voltage at the circuit 145and output the resulting voltage measurements to the test circuit 1040.The test circuit 1040 may keep track of the minimum (smallest) measuredvoltage received from the voltage detector 1030. After the circuit 145completes operations across the frequency range, the test circuit 1040may record the minimum measured voltage for the resistance value. Theminimum measured voltage provides an indication of the peak impedancefor the resistance value, in which a higher minimum measured voltage isindicative of a lower peak impedance and a lower minimum measuredvoltage is indicative of a higher peak impedance.

The test circuit 1040 may repeat the above procedure for each resistancevalue to determine the minimum measured voltage for each resistancevalue. The test circuit 1040 may then select the resistancecorresponding to the highest minimum measured voltage, and hence lowestpeak impedance. Alternatively, the test circuit 1040 may select any oneof the resistance values corresponding to a minimum measured voltagethat is equal to or greater than a minimum-voltage threshold. Theminimum-voltage threshold may correspond to a minimum supply voltageneeded for one or more circuits on the die 620 to function properly. Thetest circuit 1040 may then instruct the resistance controller 625 to setthe resistance of the resistor switch 615 to the selected resistancevalue.

FIG. 11 shows an exemplary plot of the minimum measured voltage as afunction of resistance of the resistor switch 615. For example, the plotmay be generated by having the resistance controller 625 sequentiallyset the resistance of the resistor switch 615 to each one of a pluralityof different resistance values, and recording the minimum measuredvoltage for each resistance. In the example shown in FIG. 11, theresistance of the resistor switch 615 is varied across a range boundedby a low resistance and a high resistance. The highest minimum measuredvoltage (and hence lowest peak impedance) occurs at an optimalresistance between the low and high resistances. In FIG. 11, the minimummeasured voltages corresponding to the low resistance, optimalresistance, and high resistance are denoted low, optimal, and high,respectively.

Although both the voltage detector 1030 and test circuit 1040 arelocated on the die 1020 in the example shown in FIG. 10, it is to beappreciated that one of the voltage detector 1030 and test circuit 1040may be external to the die 1020 or both of the voltage detector 1030 andtest circuit 1040 may be external to the die 1020.

FIG. 12 is a flow diagram illustrating a method 1200 for tuning theresistance of a resistor switch according to an embodiment of thepresent disclosure. The resistor switch (e.g., resistor switch 615) maybe on a die (e.g., die 620) and may be coupled between an external powerline (e.g., power line 112) and an external capacitor (e.g., packagecapacitor C_(pkg)). The method 1200 may be performed by the voltagedetector 1030 and the test circuit 1040.

In step 1210, the resistance of the resistor switch is sequentially setto each one of a plurality of resistance values. This may be done, forexample, by instructing a resistance controller (e.g., resistancecontroller 625) that controls the resistance of the resistor switch(e.g., resistor switch 615) to sequentially set the resistance of theresistor switch to each one of the resistance values.

In step 1220, a peak impedance is determined for each one of theresistance values. For example, the peak impedance for each resistorvalue may be determined by operating a circuit (e.g., circuit 145)coupled to the external power line across a frequency range, measuring asupply voltage of the circuit as the circuit is operated across thefrequency range, and recording a minimum measured voltage. In thisexample, the minimum measured voltage for each resistance value maycorrespond to the peak impedance for the resistance value.

In step 1230, one of the resistance values is selected based on thedetermined peak impedances. For example, the resistance valuecorresponding to a lowest one of the determined peak impedances may beselected. For the example in which the peak impedance for eachresistance value is determined based on the minimum measured voltage forthe resistance value, the resistance value corresponding to the highestminimum measured voltages may be selected. Alternatively, any one of theresistance values corresponding to a minimum measured voltage that isequal to or greater than a minimum-voltage threshold may be selected.The method may further include setting the resistance of the resistorswitch to the selected resistance value.

Thus, embodiments of the present disclosure allow the resistance of theresistor switch 615 to be tuned for a particular die. The ability totune the resistance of the switch resistor 615 provides an advantageover using a fixed resistor (e.g., metal resistor) to control theresistance of the RLC loop 652. This is because the resistance of afixed resistor typically has to be determined before fabrication. Forexample, the resistance of a metal resistor needs to be determinedbefore fabrication in order to specify the dimensions of the metalresistor. However, the resistance needed to achieve a desired reductionin the peak impedance for a particular die may not be known until afterthe die has been fabricated due to process variations and/or otherfactors. As a result, the resistance of the fixed resistor may notachieve the desired reduction in the peak impedance. Embodiments of thepresent disclosure overcome the above drawbacks of using a fixedresistor. This is because the resistance of the resistor switch 615 canbe tuned to a resistance value that achieves a desired reduction in thepeak impedance for a particular die after fabrication.

Although FIG. 6 shows one power-gated circuit 145 for ease ofillustration, it is to be appreciated that a die may include two or morepower-gated circuits. In this regard, FIG. 13 shows a die 1320comprising a first power-gated circuit 1345 a and a second power-gatedcircuit 1345 b according to an embodiment of the present disclosure. Forexample, the first and second circuits 1345 a and 1345 b may comprisefirst and second processors (e.g., central processing units (CPUs)) of amulti-core system on a chip (SoC). The first circuit 1345 a has diecapacitance modeled as die capacitor Ca_(die) and resistance modeled asdie resistor Ra_(die), and the second circuit 1345 b has capacitancemodeled as die capacitor Cb_(die) and resistance modeled as die resistorRb_(die),

The die 1320 also includes a power manager 1325 and first and secondpower-gating switches 1340 a and 1340 b. The first power-gating switch1340 a is coupled between the power grid and the first circuit 1345 a,and the second power-gating switch 1340 b is coupled between the powergrid and the second circuit 1345 b. The power manager 1325 is configuredto manage power to each of the first and second circuits 1345 a and 1345b by controlling the respective power-gating switch 1340 a and 1340 b.For example, the power manager 1325 may connect the first circuit 1345 ato the power grid when the first circuit 1345 a is active by turning onthe first power-gating switch 1340 a, and may disconnect the firstcircuit 1345 a from the power grid when the first circuit 1345 a isinactive (e.g., idle) by turning off the first power-gating switch 1340a. Similarly, the power manager 1325 may connect the second circuit 1345b to the power grid when the second circuit 1345 b is active by turningon the second power-gating switch 1340 b, and may disconnect the secondcircuit 1345 b from the power grid when the second circuit 1345 b isinactive (e.g., idle) by turning off the second power-gating switch 1340b.

The power manager 1325 may independently control the first and secondpower-gating switches 1345 a and 1345 b, and therefore independentlypower gate the first and second circuits 1345 a and 1345 b. For example,the power manager 1325 may independently power gate the first and secondcircuits 1345 a and 1345 b based on usage of the first and secondcircuits 1345 a and 1345 b. In the example shown in FIG. 13, eachpower-gating switch 1345 a and 1345 b comprises a PMOS transistor. Inthis example, the power manager 1325 may turn on one of the power-gatingswitches 1345 a and 1345 b by driving the gate of the switch low, andmay turn off one of the power-gating switches 1345 a and 1345b bydriving the gate of the switch high.

The die 1320 includes the resistor switch 615 between the power grid andthe package capacitor C_(pkg). In the example shown in FIG. 13, thefirst circuit 1345 a, the resistor switch 615 and the package capacitorC_(pkg) form a first RLC loop 1352 a, and the second circuit 1345 b, theresistor switch 615 and the package capacitor C_(pkg) form a second RLCloop 1352 b. The first and second RLC loops 1352 a and 1352 b share theresistor switch 615. Thus, the resistor switch 615 may be used to adjustthe resistance of each RLC loop 1352 a and 1352 b.

Although the first and second RLC loops 1352 a and 1352 b share theresistor switch 615, the first and second RLC loops 1352 a and 1352 bmay have different resistances. This is because the first and secondcircuits 1345 a and 1345 b connect to the power grid at differentlocations. As a result, the amount of grid resistance in each RLC loopis different. In FIG. 13, the grid resistance between the first andsecond circuits 1345 a and 1345 b is modeled as grid resistor R_(grid).The grid resistance between the first circuit 1345 a and the resistorswitch 615 and the grid resistance between the second circuit 1345 b andthe power supply 110 are not shown in FIG. 13 for ease of illustration.

Because the first and second RLC loops 1352a and 1352b have differentresistances, the peak impedance seen by each of the first and secondcircuits 1345 a and 1345 b may be minimized at a different resistancesetting of the resistor switch 615. In this regard, the resistance ofthe resistor switch 615 may be separately tuned for each of the firstand second circuits 1345 a and 1345 b to determine a resistance valuefor each circuit.

For example, the first circuit 1345 a may be connected to the power gridby the first power-gating switch 1340 a with the second circuit 1345 bdisconnected from the power grid. The resistance of the resistor switch615 may then be tuned by performing any one of the tuning proceduresdiscussed above to determine a resistance value for the first circuit1345 a. Similarly, the second circuit 1345 b may be connected to thepower grid by the second power-gating switch 1340 b with the firstcircuit 1345 a disconnected from the power grid. The resistance of theresistor switch 615 may then be tuned by performing any one of thetuning procedures discussed above to determine a resistance value forthe second circuit 1345 b. The resistance values for the first andsecond circuits 1345 a and 1345 b may be stored in memory on the die1320.

In one embodiment, the resistance value for each circuit 1345 a and 1345b may be determined based on the resistance value corresponding to thehighest minimum measured voltage (and hence lowest peak impedance) foreach circuit 1345 a and 1345 b. In this regard, FIG. 14 shows minimummeasured voltage 1410 a as a function of resistance for the firstcircuit 1345 a, and minimum measured voltage 1410 b as a function ofresistance for the second circuit 1345 b. As shown in FIG. 14, thehighest minimum measured voltage (and hence lowest peak impedance) forthe first circuit 1345 a occurs at a resistance value of R_(a), and thehighest minimum measured voltage (and hence lowest peak impedance) forthe second circuit 1345 b occurs at a resistance value of R_(b). Thus,in this example, the resistance value for the first circuit 1345 a isR_(a) and the resistance value for the second circuit 1345 b is R_(b).In FIG. 14, the minimum measured voltages corresponding to resistancevalues R_(a) and R_(b) are denoted optimal(a) and optimal(b),respectively.

A resistance value for the case where both circuits 1345 a and 1345 bare active may also be determined. For example, the resistance value forthis case may be a resistance value that is between (e.g., midwaybetween) the resistance values for the first and second circuits 1345 aand 1345 b. Referring to FIG. 14, in another example, the resistancevalue for this case may be a resistance value of R_(ab) at which theminimum measured voltage for both circuits 1345 a and 1345 b is highest.In FIG. 14, the minimum measured voltage corresponding to the resistancevalue of R_(ab) is denoted optimal(ab).

After the resistance values for the different use cases have beendetermined, the resistance controller 625 may set the resistance of theresistor switch 615 depending on the activity of the circuits 1345 a and1345 b. For example, when the first circuit 1345 a is active and thesecond circuit 1345 b is inactive, the resistance controller 625 may setthe resistance of the resistor switch 615 to the resistance value ofR_(a). When the second circuit 1345 b is active and the first circuit1345 a is inactive, the resistance controller 625 may set the resistanceof the resistor switch 615 to the resistance value of R_(b). When bothcircuits 1345 a and 1345 b are active, the resistance controller 625 mayset the resistance of the resistor switch 615 to the resistance value ofR_(ab). In this example, the resistance controller 625 may receive asignal from the power manager 1325 indicating which circuits 1345 a and1345 b are active at a given time.

It is to be appreciated that embodiments of the present disclosure arenot limited to a die with two power-gated circuits, and may beimplemented on a die with three or more power-gated circuits. Forexample, embodiments of the present disclosure may be implemented on adie with three power-gated circuits labeled circuit A, circuit B andcircuit C. In this example, a resistance value for each individualcircuit may be determined. A resistance value may also be determined foreach one of the following cases: circuits A and B are active and circuitC is inactive, circuits A and C are active and circuit B is inactive,circuits B and C are active and circuit A is inactive, and circuits A, Band C are all active. In this example, after the resistance values forthe different cases are determined, the resistance controller 625 mayadjust the resistance of the resistor switch 615 according to theactivity of the circuits. For example, if circuits A and B are activeand circuit C in inactive, then the resistance controller 625 may setthe resistance of the resistor switch 615 to the resistance valuedetermined for the case where circuits A and B are active and circuit Cis inactive. The resistance controller 625 may receive a signal from thepower manager 1325 indicating the activity of the circuits at a giventime.

In one embodiment, the resistance controller 625 may selectivelydisconnect the package capacitor C_(pkg) from the power line 112 byopening the resistor switch 615. For example, the resistance controller625 may disconnect the package capacitor C_(pkg) from the power line 112when the power line 112 is powered down and reconnect the packagecapacitor C_(pkg) to the power line 112 after the power line 112 hasbeen powered back on. This allows the package capacitor C_(pkg) toretain charge when the power line 112 is powered down. As a result, whenthe power line 112 is powered back on, the amount of charge needed fromthe power supply 110 to recharge the package capacitor C_(pkg) can besignificantly reduced (assuming the package capacitor C_(pkg) has lowleakage). In contrast, when the power line 112 in FIG. 1 is powereddown, the package capacitor C_(pkg) is discharged (the voltage of thepackage capacitor C_(Pkg) collapses). In this embodiment, the resistancecontroller 625 may receive a signal from the power manager 1325indicating when the power line 112 is powered up and powered down.

FIG. 15 is a flow diagram illustrating a method 1500 for adjusting aresistance on a die according to an embodiment of the presentdisclosure. The resistance is between an external power line (e.g.,power line 112) and an external capacitor (e.g., package capacitorC_(pkg)). The external power line may also refer to one or moreconduction paths through the package 617 that couple power from thepower supply 110 to the die 620.

In step 1510, the resistance is set to a first resistance value if afirst circuit on the die is active and a second circuit on the die isinactive. For example, the resistance may be provided by a resistorswitch (e.g., resistor switch 615) coupled between the external powerline (e.g., power line 112) and the external capacitor (e.g., packagecapacitor C_(pkg)), and the resistance of the resistor switch may be setto the first resistance value.

In step 1520, the resistance is set to a second resistance value if thefirst circuit is inactive and the second circuit is active. For example,the resistance may be provided by the resistor switch, and theresistance of the resistor switch may be set to the second resistancevalue. The method 1500 may optionally include setting the resistance toa third resistance value if both the first and second circuits areactive.

As discussed above, the power manager 1325 may disconnect the firstcircuit 1345 a from the power grid when the first circuit 1345 a isinactive (e.g., idle) to reduce power leakage. The power manager 1325does this by turning off the first power-gating switch 1340 a. In theinactive state, the die capacitor Ca_(die) of the first circuit 1345 ais discharged. When the first circuit 1345 a is needed, the powermanager 1325 may reconnect the first circuit 1345 a to the power grid byturning on the first power-gating switch 1340 a. When the firstpower-gating switch 1340 a is first turned on, the first circuit 1345 amay consume a large amount of charge from the power grid to charge upthe die capacitor Ca_(die). The transfer of charge causes the supplyvoltage at the power grid to droop. This is because the power supply 110is not able to supply charge fast enough to the power grid to preventthe droop due to the PDN inductance between the power supply 110 and thedie 1320. The voltage droop can potentially cause circuit elements onthe die 1320 to malfunction.

The package capacitor C_(pkg) may reduce the voltage droop by supplyingsome of the charge needed to charge up the die capacitor Ca_(die) of thefirst circuit 1345 a when the first power-gating switch 1340 a is firstturned on. The package capacitor C_(pkg) supplies charge to the diecapacitor Ca_(die) through the resistor switch 615. As a result, therate at which the package capacitor C_(pkg) is able to transfer chargeto the die capacitor Ca_(die), and hence the ability of the packagecapacitor C_(pkg) to reduce the voltage droop, depends on the resistanceof the resistance switch 615.

In this regard, FIG. 16 shows two voltage curves 1610 and 1620 at thepower grid for two different switch resistances, in which voltage curve1610 corresponds to a lower switch resistance than voltage curve 1620.In FIG. 16, the first power-gating switch 1340 a is turned on a time t0.Prior to time t0, the voltage at the power grid is approximately equalto the nominal supply voltage Vdd. When the first power-gating switch1340 a is first turned on, the voltage at the power grid droops due tocharge consumption by the die capacitor Ca_(die) of the first circuit1345 a. As shown in FIG. 16, the droop 1615 for voltage curve 1610 issmaller than the droop 1625 for voltage curve 1620. This is because theswitch resistance is lower for voltage curve 1610. As a result, thepackage capacitor C_(pkg) is able to supply charge to the die capacitorCa_(die) at a faster rate through the resistor switch 615, and thereforereduce the voltage droop by a greater amount.

As also shown in FIG. 16, both voltage curves 1610 and 1620 oscillate atthe resonant frequency of the RLC loop 1352 a. The oscillations aredampened by the resistance of the resistor switch 615. The higher theswitch resistance, the more the oscillations are dampened. As a result,the oscillations die out faster for voltage curve 1620 compared withvoltage curve 1610.

Thus, each voltage curve 1610 and 1620 has a desirable property. Voltagecurve 1610 has a smaller initial voltage droop than voltage curve 1620,while the oscillations of voltage curve 1620 are dampened out at afaster rate than voltage curve 1610.

In one embodiment, the resistance controller 625 dynamically adjusts theresistance of the resistance switch 615 during power up of the firstcircuit 1345 a to achieve a voltage curve having the desirableproperties of both voltage curves 1610 and 1620. More particularly, whenthe power-gating switch 1340 a is first turned on, the resistancecontroller 625 may set the resistance of the resistance switch 615 to afirst resistance value. After the initial voltage droop, the resistancecontroller 625 may set the resistance of the resistance switch 615 to asecond resistance value, in which the second resistance value is greaterthan the first resistance value. The first resistance value allows thepackage capacitor C_(pkg) to supply charge to the die capacitor Ca_(die)faster (thereby reducing the initial voltage droop by a larger amount),while the second resistance value (which is greater than the firstresistance value) dampens out oscillations at a faster rate.

In one embodiment, the resistance controller 625 may monitor the voltageat the power grid during power up of the first circuit 1345 a, anddynamically adjust the resistance of the resistor switch 615 based onthe detected voltage. For example, the resistance controller 625 may setthe resistance of the switch resistor 615 to the first resistor valuewhen the voltage at the power grid falls below a first voltagethreshold, indicating the start of the initial voltage droop. The firstvoltage threshold may be a voltage that is slightly below the nominalsupply voltage Vdd. The resistance controller 625 may then set theresistance of the switch resistor 615 to the second resistor value whenthe voltage at the power grid rises above a second voltage threshold,indicating that the initial voltage droop has passed. The second voltagethreshold may be approximately equal to Vdd. The first and secondvoltage thresholds may have the same values or different values. In theexample shown in FIG. 16, the resistance controller 625 may set theresistance of the resistor switch 615 to the first resistance valueduring a time period between t0 and t1 to reduce the initial droop.

FIG. 17 shows exemplary circuitry for dynamically adjusting theresistance of the resistor switch 615 during power up of the firstcircuit 1345 a according to an embodiment of the present disclosure. Inthis embodiment, the die 1720 includes a voltage detector 1730 coupledto the power grid. The voltage detector 1730 is configured to measurethe voltage at the power grid and to output the measured voltage to theresistance controller 625.

When the first circuit 1345 a is to be powered up from an inactive stateto an active state, the power manager 1325 may send a signal to theresistance controller 625 indicating that the first circuit 1345 a isabout to be powered up. In response, the resistance controller 625 maymonitor the measured voltage from the voltage detector 1730. When themeasured voltage falls below the first voltage threshold, the resistancecontroller 625 may set the resistance of the resistor switch 615 to thefirst resistance value. The resistance controller 625 may continue tomonitor the measured voltage, and, when the measured voltage rises abovethe second voltage threshold, the resistance controller 625 may set theresistance of the resistor switch 615 to the second resistance value.After the oscillations have been sufficiently reduced to begin activeoperation of the first circuit 1345 a, the resistance controller 625 mayset the resistance of the resistor switch 615 to the resistance valuedetermined above for the active state (e.g., resistance value determinedusing the method 1200 in FIG. 12).

Alternatively, the resistance controller 625 may set the resistance ofthe resistor switch 615 to the second resistance value after apredetermined time delay from the time that the resistance of theresistance switch 615 is set to the first resistance value. The timedelay may be based on an estimation of the time duration of the initialvoltage droop.

FIG. 18 shows an exemplary implementation of the voltage detector 1730according to an embodiment of the present disclosure. In thisembodiment, the voltage detector 1730 comprises an odd number ofinverters 1830-1 to 1830-3 coupled in series, in which the output of thelast inverter 1830-3 is coupled to the input of the first inverter1830-1. As a result, the inverters 1830-1 to 1830-3 form a ringoscillator 1835, in which the oscillation frequency of the ringoscillator 1835 is a function of the delays of the inverters 1830-1 to1830-3.

The inverters 1830-1 to 1830-3 are biased by a voltage at the input 1810of the voltage detector 1730, which is coupled to the power grid. Thus,the inverters 1830-1 to 1830-3 are biased by the voltage at the powergrid. The delays of the inverters 1830-1 to 1830-3 are a function of thebias voltage of the inverters 1830-1 to 1830-3, and hence are a functionof the voltage at the power grid. Since the oscillation frequency of thering oscillator 835 is a function of the delays of the inverters 1830-1to 1830-3, the oscillation frequency of the ring oscillator is afunction of the voltage at the power grid, and may therefore be used tomeasure the voltage at the power grid.

In this regard, the voltage detector 1730 comprises a counter 1840coupled to the ring oscillator 1835. The counter 1840 is configured tocount the number of oscillations of the ring oscillator 1835 over aperiod of time, and output the count value to the resistance controller625 at output 1820. The count value is a function of the oscillationfrequency of the ring oscillator, and hence a function of the voltage atthe power grid. Thus, the count value from the counter 1840 provides ameasurement of the voltage at power grid.

It is to be appreciated that the resistance of the resistor switch 615may also be dynamically adjusted during power up of the second circuit1345 b. For example, the resistance controller 625 may monitor thevoltage at the power grid during power up of the second circuit 1345 b,and dynamically adjust the resistance of the resistor switch 615 basedon the detected voltage in a manner similar to that described above forthe first circuit 1345 a.

Those skilled in the art would appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection may be properly termed acomputer-readable medium to the extent involving non-transient storageof transmitted signals. For example, if the software is transmitted froma website, server, or other remote source using a coaxial cable, fiberoptic cable, twisted pair, digital subscriber line (DSL), or wirelesstechnologies such as infrared, radio, and microwave, then the coaxialcable, fiber optic cable, twisted pair, DSL, or wireless technologiessuch as infrared, radio, and microwave are included in the definition ofmedium, to the extent the signal is retained in the transmission chainon a storage medium or device memory for any non-transient length oftime. Disk and disc, as used herein, includes compact disc (CD), laserdisc, optical disc, digital versatile disc (DVD), floppy disk andblu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveshould also be included within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples described herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. An apparatus, comprising: a capacitor; and a die,the die comprising: a resistor switch coupled between a power line andthe capacitor, wherein the resistor switch has an adjustable resistance,and the power line and the capacitor are both external to the die; acircuit configured to receive power from the power line; and acontroller configured to open the resistor switch if the power line ispowered down.
 2. The apparatus of claim 1, further comprising a package,wherein the die is mounted on the package and the capacitor is in thepackage.
 3. The apparatus of claim 2, wherein the capacitor comprises amultilayer ceramic capacitor embedded in the package.
 4. The apparatusof claim 1, further comprising a package, wherein the die and thecapacitor are mounted on the package.
 5. The apparatus of claim 1,wherein the die further comprises a power-gating switch coupled betweenthe circuit and the power line, wherein the power-gating switch isconfigured to power gate the circuit.
 6. An apparatus, comprising: acapacitor; and a die, the die comprising: a resistor switch coupledbetween a power line and the capacitor, wherein the resistor switch hasan adjustable resistance, and the power line and the capacitor are bothexternal to the die; a circuit configured to receive power from thepower line; and a resistor controller configured to dynamically adjustthe resistance of the resistor switch during power up of the circuitfrom an inactive state to an active state.
 7. The apparatus of claim 6,wherein the die further comprises a voltage detector configured todetect a voltage on a power grid of the die, and wherein, during powerup of the circuit, the resistor controller is configured to set theresistance of the resistor switch to a first resistance when thedetected voltage falls below a first voltage threshold, and to set theresistance of the resistor switch to a second resistance value when thedetected voltages rises above a second voltage threshold.
 8. Theapparatus of claim 6, further comprising a package, wherein the die ismounted on the package and the capacitor is in the package.
 9. Theapparatus of claim 8, wherein the capacitor comprises a multilayerceramic capacitor embedded in the package.
 10. The apparatus of claim 6,further comprising a package, wherein the die and the capacitor aremounted on the package.
 11. The apparatus of claim 6, wherein the diefurther comprises a power-gating switch coupled between the circuit andthe power line, wherein the power-gating switch is configured to powergate the circuit.
 12. A method for tuning a resistance of a resistorswitch on a die, wherein the resistor switch is between an externalpower line and an external capacitor, the method comprising:sequentially setting the resistance of the resistor switch to each oneof a plurality of resistance values; for each resistance value,determining a peak impedance for the resistance value; and selecting oneof the resistance values based on the determined peak impedances. 13.The method of claim 12, wherein selecting one of the resistance valuescomprises selecting a resistance value corresponding a lowest one of thedetermined peak impedances.
 14. The method of claim 12, whereinselecting one of the resistance values comprises selecting a resistancevalue corresponding to a determined peak impedance that is equal to orbelow a threshold.
 15. The method of claim 14, wherein the threshold isbased on a minimum supply voltage needed for one or more circuits on thedie to function properly.